1. Field of the Invention
The invention relates to a resistance projection welder for welding together first and second work pieces, one of which has a series of projections contacting the other work piece to establish electrical conductivity between the work pieces and form the weld therebetween; and, more particularly, sensing the relative orientation between the first and second work pieces in determining if the orientation is proper.
2. Related Art
Resistance welding is a well-known technique for welding together two work pieces, especially sheet metal and similar materials. A typical resistance welder comprises spaced electrodes between which are positioned the two work pieces to be welded together. The electrodes are brought into contact with the two work pieces and clamp the work pieces together. An electric current is then passed from one electrode to the other through the work pieces. The resistance of the work pieces generates sufficient heat to render the metal semi-molten, forming a weld nugget between the two work pieces and thereby welding them together.
In a resistance projection welding process, one of the work pieces has a series of projections extending from a body of the one work piece. The work pieces are oriented such that the projections from the one work piece contact the other work piece to conductively couple together the work pieces. The electrodes clamp the first and second work pieces together. Electric current applied through the work piece travels through the projections, heating the projections and the material of the other work piece in contact with the projections. The projections form a weld nugget along with the corresponding contacting portions of the other work piece. In essence, the projections provide additional material to form a weld nugget between the first and second work pieces.
In a resistance projection welding process, it is very important for the work pieces to be properly oriented. That is, it is very important for the projections to extend from the one work piece body to the other work piece, instead of the body of the one work piece contacting the other work piece and the projections extending away from the other work piece. Since many resistance projection welding processes produce high volume output, it is possible for the work pieces to be improperly oriented. It is desirable to have a simple, reliable and reproducible method for determining the proper orientation of the work pieces prior to their welding together.
One previous attempt to determine the orientation of the work pieces in a resistance projection welding process relied on applying a source of pressurized air to the electrode contacting the work piece with the projections and measuring the back pressure in the air system. If the work pieces were properly aligned, the work piece with the projections would substantially block the flow of air exiting the air system and thereby increase the back pressure in the air system. Alternatively, if the work piece with the projections was improperly aligned, the projections would form an air gap relative to the electrode permitting the escape of air from the air system and reducing the back pressure within the air system. This solution proved inadequate as the system was not capable of discerning fine changes in back pressure indicative of proper or improper orientation of the work pieces, resulting in either two few or two many indications of improper orientation.
It is still desirable to have a resistance projection welding apparatus and method capable of identifying when the work pieces are properly oriented prior to their welding. However, the apparatus and method must be capable of repeatedly and accurately discerning the orientation of the work pieces.